Embodiments described relate to die attachment to a package substrate. In particular, embodiments described herein relate to a semiconductor package which includes a package substrate having a surface to accommodate stacked dice.
In the fabrication of microchips or dice, semiconductor wafers are processed and sliced into individual dice. The dice may then be used in a wide variety of devices. For example, a die may be used in an electronic device by being electronically coupled to a printed circuit board (PCB) of the device. However, prior to such an electronic coupling, packaging takes place. Packaging is the manner by which a semiconductor wafer is separated into individual dice that are then protected in various package forms. The protective packages prevent damage to the die and provide an electrical path to the circuitry of the die.
The package includes a protective package substrate having a surface to which the die is secured and electronically coupled. In many cases a second die, positioned above the first die and secured thereto, is also electronically coupled to the package substrate. The resulting semiconductor package is often referred to as being of a xe2x80x9cstackedxe2x80x9d configuration.
The stacked dice are secured to the package substrate by first aligning metal bumps of the first die above bond pads at the surface of the package substrate. The metal bumps and bond pads are soldered to one another to provide electronic coupling. An underfill material, generally an epoxy adhesive, is then applied, in liquid form, between the first die and the surface of the package substrate, filling the space there between. The underfill material is then cured to secure and stabilize the first die to the substrate. The second die of the stacked configuration has been similarly secured to the first die. For example, an epoxy or other adhesive material may be used to secure a lower surface of the second die to the first die.
Electrical coupling may then be provided between metal bumps, referred to here as metal contacts, at an upper surface of the second die and certain bond pads at the surface of the substrate. This is accomplished by wire bonding. For example, metal wires, often of gold, are ran from the metal bumps of the second die to the certain bond pads at the surface of the substrate. The certain bond pads at the surface of the substrate to couple to the second die are referred to here as bond fingers. The bond fingers are bond pads that are not aligned below the first die and have not been coupled to the metal bumps of the first die. Rather, they are adjacent the other bond pads at the surface of the substrate, to be left available for wire bonding to the metal contacts of the second die. However, as described below, dispensing underfill material to secure the first die to the substrate, may interfere with the coupling of the bond fingers to the metal contacts of the second die.
Dispensing of underfill material is accomplished by a heated dispensing needle. The dispensing needle is positioned to deliver underfill material between the first die and the package substrate. The exact position of the needle may greatly affect the resulting performance of the first die. For example, if the first die is contacted by the dispensing needle, it may be severely damaged. On the other hand, the further the needle is from the first die, the less control is maintained over the spreading or flow of the underfill material. Additionally, the degree of precision required in delivering underfill material becomes more and more difficult to attain with complete accuracy, especially as packaging dimensions continue to diminish in size. However, it is important that the bond fingers remain free of underfill material for subsequent wire bonding to the metal contacts of the second die.